1. Field of the Invention:
The present invention relates generally to a control system for controlling an engine for a motor vehicle, and particularly to a control system for controlling an actuator which controls operation of an internal combustion engine to follow-up a target value varying continuously depending on the operation condition of the engine.
2. Prior Art Statement:
During cruising of a vehicle, the rotation speed of the engine is changed greatly and the loading applied on the engine is also changed within a wide range. In consideration of the foregoing, it has been proposed to detect the operation conditions of the engine and to open or close a variety of control valves depending on the result of detection so as to achieve optimum control of the engine. For example, Japanese Patent Laid-Open Publication No. 126222/1987 teaches a system wherein an exhaust gas control valve is disposed at a vicinity of the downstream open end of the exhaust pipe, and the exhaust gas control valve is fully opened to utilize the kinetic effect of the exhaust system at the maximum extent so as to increase the output of the engine when the engine is operated within its high speed range. The exhaust gas control valve is closed to about one half of the full open angle, when the engine is operated within its medium speed range, to prevent formation of trough of torque due to the reverse effect of kinetics in the exhaust system. Another proposal has been made to change the effective length of the exhaust pipe by the provision of control valves on the connection pipes connecting the plural exhaust pipes and by opening or closing the control valves depending on the change in rotation speed of the engine.
During the course of opening and closing these valves so that the control valves take the desired opening angles in compliance with the change in operation condition of the engine, there might arise a trouble where any one of the motors or actuators serving to open and close the control valves rotates in the obverse and reverse directions alternatively and repeatedly as shown in FIGS. 4(A) and 4(B). FIG. 4(A) shows a case where the loading applied on the actuator is relatively large and some slack is present along the power transmission elements. In this case, the actuator is rotated in the reverse direction due to the tension in the wire or other power transmission elements after the electric current flowing through the actuator is cut off when the opening angle of a control valve takes a up-to-date opening angle .theta. which is included in the allowable range spanning a constant width determined while taking the target value as a central value. The opening angle of the valve is thus deviated from the allowable range .circle. by such a rotation of the actuator, and an electric current energizing the actuator is flown again. As a result, undesirable alternating rotation of the actuator in the obverse and reverse directions are repeated. On the contrary, FIG. 4(B) shows a case where the loading applied on the actuator is too small and the inertia of the actuator is excessively large. In such a case, the actuator is stopped after the control valve rotates beyond the allowable range .circle. of the target valve. Then, the actuator is energized again to rotate the valve in the reverse direction and stopped after the control valve rotates beyond the opposing limit of the allowable range .circle. . Endlessly recurring operations are thus repeated.
The resulting endless alternating rotations of the actuator in the obverse and reverse directions lead to repeated flowing of high electromotive currents through the actuator to increase excessive temperature raise thereof, whereby the lifetime of the actuator is reduced.